1. Field of the Invention
The present invention relates to plate heat and mass exchangers for indirect evaporative coolers. In particular, the present invention relates to such plates having edge extensions for enhanced fluid removal.
2. Discussion of the Background Art
Indirect evaporative cooling is a method of cooling a fluid stream; usually air, by evaporating a cooling liquid, usually water, into a second air stream while transferring heat from the first air stream to the second. The method has certain inherent advantages compared to conventional air conditioning: low electricity requirements, relatively high reliability, and the ability to do away with the need for refrigerants such as R-134 and all the disadvantages they entail.
U.S. Pat. No. 6,581,402 shows a number of embodiments for indirect evaporative cooling using plate apparatus. FIG. 1 (Prior art) shows a perspective and schematic representation of two plates showing the wet side channels formed by the wet sides of a first and a second plate opposing each other, with their passages oriented in the same general area and illustrating the working gas entering on the dry side, passing through the passages and into the wet side channels. The product fluid is separated from the working gas as they pass along the dry side of the first and second plates. Additional plates form a stack, and adjacent plates have their dry sides facing each other. Thus, the stack of plates would have every odd plate oriented with its dry side facing the same direction and opposite of all even plates.
The invention of U.S. Pat. No. 6,581,402 provides an indirect evaporative cooler having cross flowing wet and dry channels on opposite sides of a plurality of heat exchange plates which allow heat transfer through the plates. The plates include edge extensions to facilitate the removal of water (or similar evaporative fluid) and dissolved minerals from the plates.
For purposes of both U.S. Pat. No. 6,581,402 and the present application, we wish to define certain terms:    1. Heat transfer surface or heat exchange surface has many configurations. All are encompassed within the subject of this disclosed invention with appropriate adjustment to the wetting and flows as are well known in the industry. For illustration we make use of a plate configuration.    2. Wet side or wet portion of the heat exchange surface means that portion having evaporative liquid on or in its surface, thus enabling evaporative cooling of the surface and the absorption of latent heat from the surface.    3. Dry side or dry portion of the heat exchanger means that portion of the heat exchanger surface where there is little or no evaporation into the adjacent gas or fluid. Thus, there is no transfer of vapor and latent heat into adjacent gases. In fact, the surface may be wet but not with evaporative fluid or wet by condensation, but no evaporation exists.    4. Working stream or working gas stream is the gas flow that flows along the heat exchange surface on the dry side, passes through the passages in the surface to the wet side and picks up vapor and by evaporation, taking latent heat from the heat exchange surface and transporting it out into the exhaust. In some embodiments, the working stream may be disposed of as waste and in others it may be used for special purposes, such as adding humidity or scavenging heat.    5. Product stream or product fluid stream is the fluid (gas, liquid or mixture) flow that passes along the heat exchange surface on the dry side and is cooled by the absorption of heat by the working gas stream on the wet side absorbing latent heat by the evaporation in the wet area.
The plate also has passageways or perforations or similar transfer means between the dry side of the plate and the wet side in defined areas providing flow from the dry working channels to the working wet channels in which direct evaporative cooling takes place.
The method of the invention makes use of the separation of a working gas flow (that is used to evaporate liquid in the wet channels and thus to cool the wet surface of the heat exchanger plate) from the product fluid flow, flowing through dry product channels and dry working channels respectively on the same side of the heat exchange plate. Both give up heat to the heat exchange plate that on its obverse surface is being cooled by evaporation in the working wet channels.
The working gas flow first enters the dry working channel and then through perforations, pores or other suitable means of transfer across the barrier of the plate to the wet side and thence into the wet working channels where evaporation of liquid on the wet channel surface, cools this plate.
The dry product channels are on the dry side of this plate. The plate is of a thin material to allow easy heat transfer across the plate and thus to readily allow heat to transfer from the dry product channel to the wet working channel. This is one basic unit or element of the invention illustrating the method of the separation of working gas flows to indirectly cool the separate product fluid by evaporative cooling.
Many evaporative cooling embodiments include a wicking material for distributing the water or other evaporative liquid over the plate wet side. See, for example, FIG. 7 of U.S. Pat. No. 6,581,402, wherein a wicking material 7 distributes the evaporative liquid along wet side channels 5. Plates 6 form a “V-shape” in the embodiment of FIG. 7. Water also evaporates better from a wicking surface that from a water surface, as the wick material breaks down the surface tension of the water.
Wicking up a vertical surface will insure no excess water on the plate surface but also limits the height of the plate that can be used. Wicking water down a surface aided by gravity may be good from a wetting perspective if the amount of water does not exceed what the wick can transport. Wicking in a more horizontal direction can allow a vertical reservoir wetting system such as shown in U.S. Pat. No. 6,705,096. There are some plate heat and mass exchanger applications that require a more innovative geometry that corresponds to a more complicated thermodynamic design that again require a more horizontal application such as U.S. Pat. No. 6,581,402. In all cases creating a means to insure that the wick will not be over run by water is desired.
The indirect evaporative cooler of U.S. Pat. No. 6,581,402 works well. But a disadvantage inherent in the design has been found in use. Sloping the plates to allow gravity to help pull water through the wick helped to remove excess liquid and washing minerals off the plates. However, the closely spaced heat exchanger plates, with wicking surfaces facing each other, allowed water to build up in the channels. This buildup was caused by the surface tension of the water adhering the edge of the plates. For example, given two horizontal plates in parallel, a drip from the top plate would hang down and adhere to a drip on the lower at the plate edges. Water would then back up from the edges of the plates on the wick surfaces giving two detrimental effects. First the surface water significantly reduced the heat transfer rate and thus the cooling of the fluid on the opposite side of the plate. Second, this over wetting between the plates caused an uneven airflow distribution across the wet plates and therefore uneven cooling of the fluid to be cooled on the opposite side of the plates.
As water in the wet channels is evaporated any dissolved minerals that were in the water are left behind. Even if not all of the water is evaporated away, when the minerals in the water become too concentrated they deposit on any surface they come into contact with. Such deposited minerals present a long-term problem, as they build up and eventually impede the flow of water, particularly in the wick material. Portions of the plate are no longer thoroughly wetted, and heat exchange efficiency drops.
Therefore, a need remains in the art for apparatus and methods for drawing excess liquid and minerals away from the heat exchanging portion of the plate, and removing them from the plate.